1. Field of the Invention
The present invention pertains to a technique which makes optimal usage of satellite resources and provides switched ISDN connections to the end users via any satellite network, at a Quality Of Service (QOS) level which is comparable to that of a terrestrial network. More specifically, the invention relates to techniques that can be implemented in a cost-effective manner while providing wideband (N.times.64 kbps, N=1 to 30) switched ISDN connections via a satellite network with acceptable QOS to the end-users.
2. Brief Discussion of Related Art
In an attempt to standardize digital communications links, media and interfaces, the Internation Telecommunications Union (ITU), a communications standard group that is part of the United Nations, has defined a recommendation for a worldwide Integrated Services Digital Network (ISDN) capable of handling voice and data over copper wires, fiberoptics, satellite channels and other implementations of future technologies. For example, ISDN uses a single digital transmission network to provide services such as voice, text, facsimile, videotex, and video, both switched and nonswitched, and both circuit-and packet-mode. ISDN services are of two types; bearer services, which provide transmission of signals between user-network interfaces, and teleservices, which include terminal equipment functions.
ISDN channels include the B channel, whose rate is 64 kbps accompanied by timing. It carries the user's information but not signaling information. The B channel may carry multiplexed information, but it is switched as a unit. The D channel primarily carries signaling information for circuit switching by the ISDN, but it may also be used for telemetering, alarm, and control information, and for packet-switched data.
These channels are combined into standard interfaces. The Basic Rate Interface (BRI) contains two B channels and one 16-kbps D channel. The B channels may be used independently, in different connections. At the primary rate of 1,544 or 2,048 kbps, a Primary Rate Interface (PRI) can be provided as 23 B channels and one 64-kbps D channel, or 30 B channels and one D channel.
It will be appreciated that ISDN interfaces are intended to support single ISDN terminals, multiple-ISDN-terminal installations, PBXs, LANs, private networks, information storage and processing centers, networks dedicated to a specific service, and other multiple-service networks, including ISDNs.
It should be noted that ISDNs are covered by ITU Recommendations in the I-series, including I.120, Description of ISDNs; I.210, Service Aspects of ISDNs; and I.340, ISDN Connection Types. Several recommendations cover user-network interfaces; I.410, General Aspects and Principles; I.411, Reference Configurations; and I.412, Interface Structures and Access Capabilities. Other aspects of ISDNs are covered by ITU Recommendations in the Q-series, as discussed in greater detail below.
Because such a system must encompass high-level data such as video images, computer messages, voice communications, and other information, and must also specify low-level data concerning wires, connectors, frequencies, voltages, etc., the system has been designed in "layers" with the bottom layer (layer 1) representing the physical phenomenon, and the top layer (layer 7) representing user applications. In between are layers that partition the network in terms of well-defined interfaces that range from the interface at the bottom (physical) layer, over which physical signals are passed, up an increasingly abstract hierarchy to the most general, "application" layer which represents the user's desired task or application that makes use of the digital (ISDN) communications network. The ITU layers are rigorously defined at the interfaces between layers, and the messages that flow between the layers are also specified. It is important to note that the implementation of the layers is not specified, leaving complete freedom to the designer of the layered communication system. In use, messages flow down from the top layers to the physical layer, across the network, and up to the "peer" layer at the destination.
While the seven-layer scheme is designed to allow any computer to communicate with any other computer, regardless of make or manufacture, the top layers are not absolutely essential to successful communications across the network. The bottom three layers, i.e., the Physical (1), Data Link (2), and Network (3) layers, are essential, and must exist where any device or system is capable of communicating across the ISDN. See U.S. Pat. No. 5,337,403 to Klingman.
Although ISDN standards have recently been developed for the provision of a multitude of services via common signaling and interface procedures, it is expected that the global infrastructure set up by ISDN will play a significant role in the communications world. Satellite communications will be part of that role.
Satellite communication networks have unique strengths. Satellite channels are agile in the sense that they can be accessed by a widely dispersed community of users. This is in contrast to terrestrial networks where the communication channels are fixed and can be accessed by only those users who are directly connected to those channels. Also, satellite channels are inherently multipoint/broadcast in nature. Thus, any earth station in the uplink beam coverage of the satellite channel can transmit signals on that channel and it can be received by all earth stations in the downlink beam coverage of that channel. However, certain data protocols degrade over satellite links when used with certain parameters and procedures.